全文获取类型
收费全文 | 2242篇 |
免费 | 306篇 |
国内免费 | 168篇 |
专业分类
化学 | 2385篇 |
晶体学 | 8篇 |
力学 | 25篇 |
综合类 | 22篇 |
数学 | 26篇 |
物理学 | 250篇 |
出版年
2024年 | 4篇 |
2023年 | 20篇 |
2022年 | 56篇 |
2021年 | 75篇 |
2020年 | 114篇 |
2019年 | 93篇 |
2018年 | 65篇 |
2017年 | 60篇 |
2016年 | 114篇 |
2015年 | 125篇 |
2014年 | 110篇 |
2013年 | 290篇 |
2012年 | 101篇 |
2011年 | 97篇 |
2010年 | 105篇 |
2009年 | 88篇 |
2008年 | 82篇 |
2007年 | 88篇 |
2006年 | 84篇 |
2005年 | 95篇 |
2004年 | 86篇 |
2003年 | 92篇 |
2002年 | 189篇 |
2001年 | 62篇 |
2000年 | 41篇 |
1999年 | 38篇 |
1998年 | 36篇 |
1997年 | 46篇 |
1996年 | 49篇 |
1995年 | 58篇 |
1994年 | 17篇 |
1993年 | 19篇 |
1992年 | 37篇 |
1991年 | 10篇 |
1990年 | 17篇 |
1989年 | 12篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1978年 | 1篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1972年 | 4篇 |
1971年 | 1篇 |
排序方式: 共有2716条查询结果,搜索用时 46 毫秒
131.
Almaz Zagidullin Yulia Ganushevich Vasili Miluykov Oleg Sinyashin Evamarie Hey-Hawkins 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-3):238-242
Abstract The reactivity of 1-alkyl-1,2-diphospholes in cycloaddition reactions with dienes, dienophiles or 1,3-dipoles was examined. 1-Alkyl-1,2-diphospholes (2 ) exhibit dual reactivity and act as diene toward maleic acid derivatives or as dienophiles with 2,3-dimethyl-1,3-butadiene. The 1-alkenyl-1,2-diphospholes (4 ) are readily involved in intramolecular [4 + 2] cycloaddition reactions leading to cage phosphines (5 ). Interaction of 1-alkyl-1,2-diphospholes (2) with 1,3-dipolar reagents (diphenyldiazomethane and nitrones) results in formation of the bicyclic phosphiranes (8) and dimers of 1-alkyl-1,2-diphosphole oxides (9) or bicyclic phosphine oxides (10) with a β-lactam moiety depending on temperature. 相似文献
132.
Burkhard Costisella Iris Keitel 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3-4):161-165
Abstract The 1-acetoxy-1-methylthiomethanephosphoryl compounds (4) are prepared from methylthiomethanephosphoryl derivatives (1) by electrolysis in 0.6 molar sodium acetate/acetic acid solution. The reaction of 1-acetoxy compounds (4) with halogen carriers gives the 1-halogen-1-methylthiomethane-phosphoryl derivatives 5, 7 and 8. The reaction of 4a under PTC-conditions results in cleavage of the P-C-bond. Die 1-Acetoxy-1-methylthiomethanphosphoryl-Verbindungen (4) wurden aus Methylthiomethanphos-phorylderivaten (1) durch Elektrolyse in 0.6 molarer Na-acetat/Essigsäure-Lösung dargestellt. Die Reaktion der 1-Acetoxyverbindungen mit Halogenüberträgern führt zu den 1-Halogen-1-methyl-thiomethanphosphorylderivaten 5, 7 und 8. Bei Reaktion von 4a unter PTC-Bedingungen beobachtet man eine Spaltung der P-C-Bindung. 相似文献
133.
Svetlana F. Malysheva Natal’ya A. Belogorlova Nina K. Gusarova Alexander V. Artem’ev Alexander I. Albanov Boris A. Trofimov 《Phosphorus, sulfur, and silicon and the related elements》2013,188(8):1688-1693
Abstract Red phosphorus reacts with allylbenzene in the superbase system KOH-DMSO (130°C, 3 h, Ar) to give a mixture of bis(1-methyl-2-phenylethyl)phosphane (1), bis(1-methyl-2-phenylethyl)phosphane oxide (2), and 1-methyl-2-phenylethylphosphinic acid (3). Secondary phosphane oxide 2 and phosphinic acid 3 have been isolated from this mixture in 35% and 32% yield, respectively. Microwave activation of the reaction (200 W, 30 min) affords secondary phosphane 1 as the main product in 48% yield. GRAPHICAL ABSTRACT 相似文献
134.
135.
采用紫外光与臭氧结合消解水样,用离子色谱法测定水质总磷含量,色谱峰高与磷酸盐质量浓度在0-20mg/L范围内线性关系良好,检出限和测定下限分别0.008mg/L和0.03mg/L。此方法无需添加化学试剂,避免了标准分析方法中各种因素的影响,操作简单快速、省时,测定水质总磷的相对标准偏差为0.07%(n=5o对实际样品进行分析,总磷的加标回收率为94.5%~105.5%,表明该方法测定结果可靠,具有一定的应用价值。 相似文献
136.
Vincenzo Mirabello Maria Caporali Luca Gonsalvi Gabriele Manca Andrea Ienco Maurizio Peruzzini 《化学:亚洲杂志》2013,8(12):3177-3184
The selective functionalization of the polyphosphorus moiety Ph2PCH2PPh2PPPP present as a tetrahapto‐ligand in complex [Ir(dppm)(Ph2PCH2PPh2PPPP)]+ ( 1 , dppm=Ph2PCH2PPh2) was obtained by reaction of 1 with water under basic conditions at room temperature. The formation of the new triphosphaallyl moiety η3‐P3{P(O)H} was determined in solution by NMR spectroscopy, and confirmed in the solid state by a single‐crystal X‐ray structure of the stable product [Ir(κ2‐dppm)(κ1‐dppm)(η3‐P3{P(O)H})] ( 2 ). In solution, 2 has a fluxional behavior attributable to the four P atoms belonging to the tetraphosphorus moiety in 1 and exhibits a chemical exchange process involving the two PPh2 moieties of the same bidentate ligand, as determined by 1D and 2D NMR spectroscopy experiments carried out at variable temperature. The mechanism of the reaction was investigated at the DFT level, which suggested a selective attack of an in‐situ generated OH? anion on one of the non‐coordinated phosphorus atoms of the P4 moiety. The reaction then evolves through an acid‐assisted tautomerization, which leads to the final compound 2 . Bonding analysis pointed out that the new unsubstituted P3‐unit in the η3‐P3{P(O)H} moiety behaves as a triphosphallyl ligand. 相似文献
137.
Alain Valla Nathalie Méheux Dominique Cartier Benoist Valla Laurent Dufossé 《合成通讯》2013,43(2):184-190
A synthesis of a new C-15 phosphorus ylide from a C-14 enaminone is reported. This reagent, which undergoes selective 1,2- or 1,4-additions with saturated and unsaturated aldehydes, may find some synthetic use for the preparation of β-end-group retinoid derivatives. 相似文献
138.
G. Rauret R. Rubio M. Llaurado 《International journal of environmental analytical chemistry》2013,93(1-2):59-68
Abstract The destruction of the filter for metal determination in atmospheric aerosols is an important step in the analysis. Nowadays the decomposition of this kind of sample has not been completely solved, so it is interesting to study new methods for filter sample attack. In the present work the application of the combustion method in a closed flask for mixed cellulose esters filter decomposition for lead determination is proposed. For this purpose optimum operating conditions as stopper flask design, sample size, flask volume, shaking time and volume and concentration of absorbing solution are established, and accuracy and precision of the proposed method using AAS for the measurements is given. The results obtained by the closed flask combustion are compared with those found by wet attack with HNO3. The method proposed is rapid, has low reagent contamination and no loss of lead by volatilization or by amalgamation occurs. This method shows an accuracy and a precision in good agreement with the standard method. 相似文献
139.
Wei Li Chaoqin Li Zuosheng Chen Naixiu Ding Zongwei Cai 《International journal of environmental analytical chemistry》2013,93(7):691-711
Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) were analysed in 95 mountainous and park soil samples collected from 19 sites in Beijing, China. The levels of PCDD/Fs in the mountainous soils were low (0.086–0.59; mean 0.29; median 0.23 ng I-TEQ/kg). The levels in the park soils far from the city centre were a little higher (0.39–0.88, mean0.68; median 0.75 ng I-TEQ/kg). However, in the park soils near to the city centre, there were an abrupt jump in their concentration (1.7–3.2; mean 2.3, median 1.7 ng I-TEQ/kg), about ten times higher than those in the mountainous sites. Comparison of PCDD/Fs homologues and congener compositions between possible sources and samples indicated that the mountainous and park soils in Beijing had been slightly contaminated mainly by sodium pentachlorophenate, sewage sludge, atmospheric deposition and vehicle exhaust. These results have also been supported by the principal component analysis. 相似文献
140.
Dr. Fu Chen Dr. Guibin Ma Dr. Guy M. Bernard Prof. Roderick E. Wasylishen Prof. Ronald G. Cavell Dr. Robert McDonald Dr. Michael J. Ferguson 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(8):2826-2838
Several 1:1 adducts of gallium trihalides with triarylphosphines, X3Ga(PR3) (X=Cl, Br, and I; PR3=triarylphosphine ligand), were investigated by using solid‐state 69/71Ga and 31P NMR spectroscopy at different magnetic‐field strengths. The 69/71Ga nuclear quadrupolar coupling parameters, as well as the gallium and phosphorus magnetic shielding tensors, were determined. The magnitude of the 71Ga quadrupolar coupling constants (CQ(71Ga)) range from approximately 0.9 to 11.0 MHz . The spans of the gallium magnetic shielding tensors for these complexes, δ11?δ33, range from approximately 30 to 380 ppm; those determined for phosphorus range from 10 to 40 ppm. For any given phosphine ligand, the gallium nuclei are most shielded for X=I and least shielded for X=Cl, a trend previously observed for InIII–phosphine complexes. This experimental trend, attributed to spin‐orbit effects of the halogen ligands, is reproduced by DFT calculations. The signs of CQ(69/71Ga) for some of the adducts were determined from the analysis of the 31P NMR spectra acquired with magic angle spinning (MAS). The 1J(69/71Ga,31P) and ΔJ(69/71Ga, 31P) values, as well as their signs, were also determined; values of 1J(71Ga,31P) range from approximately 380 to 1590 Hz. Values of 1J(69/71Ga,31P) and ΔJ(69/71Ga,31P) calculated by using DFT have comparable magnitudes and generally reproduce experimental trends. Both the Fermi‐contact and spin‐dipolar Fermi‐contact mechanisms make important contributions to the 1J(69/71Ga,31P) tensors. The 31P NMR spectra of several adducts in solution, obtained as a function of temperature, are contrasted with those obtained in the solid state. Finally, to complement the analysis of NMR spectra for these adducts, single‐crystal X‐ray diffraction data for Br3Ga[P(p‐Anis)3] and I3Ga[P(p‐Anis)3] were obtained. 相似文献